1. Field of the Invention
This invention pertains to device structures and simple packaging schemes to realize low-cost, yet high-performance, multimode wavelength-division multiplexing (WDM) optical data links.
2. Description of the Background Art
(a) Introduction
The demand for ever faster data transmission rates (a few Gb/s up to 100 Gb/s) has attracted considerable interest in the development of high-capacity optical data links for short-haul local-area networks and fiber-to-the-desktop applications. The majority of work to date has focused on one-dimensional parallel optical data links which utilize multimode fiber ribbons with a one-data-channel-per-fiber arrangement. However, the maximum aggregate data transmission rate is limited to about 2-3 gigabytes per second and the system configuration is costly and very complicated.
One definitive solution to the bandwidth problem is to take advantage of the extra-wide bandwidth of optical fibers by employing the wavelength-division multiplexing (WDM) configuration which can significantly expand the transmission capacity by having multiple data channels in each fiber. With WDM, however, the corresponding transmitter and receiver modules must be low cost to be attractive for emerging "gigabytes-to-the-desktop" applications. A problem with WDM is that any additional complexity in device fabrication and packaging technology can dramatically increase the manufacturing cost. The availability of such low-cost multiple-wavelength emitter and detector arrays is also a key issue for the realization of ultra-high-density multiple-layer digital versatile disk technology. Obviously, the vertical-cavity device structure is the ideal candidate for WDM configurations because the resonant wavelength can be easily varied and its fiber packaging is potentially low-cost.
(b) VCSEL Emitters in Optical Data Links
With the inherent advantages of its two-dimensional configuration, efficient fiber coupling, and on-wafer testing capability, VCSEL (Vertical-Cavity Surface-Emitting Laser) structures have remained the preferred candidate for free-space interconnects and optical fiber communication since inception. Within the last decade, there have been numerous advances in epitaxial technology, device design, and processing techniques such that the performance of VCSELs has been greatly improved. Currently, major accomplishments in VCSEL technology have been demonstrated in the 0.8-1.00 .mu.m wavelength regime, where VCSELs are being incorporated into many advanced optical systems as a high-performance and yet low-cost solution for short-distance communications.
VCSELs are the ideal laser structures for the implementation of wavelength-division multiplexing (WDM) systems because the lasing wavelength can be easily varied by adjustment of the cavity length. However, due to the difficulty of achieving convenient and high-reflectivity distributed-Bragg reflector (DBR) mirrors in the longwave-length (1.3-1.55 .mu.m) regime, in-plane distributed feedback (DFB) laser arrays have been the traditional structures in long-haul WDM optical communications. On the other hand, It is difficult to make in-plane DFB lasers at 0.8-1.00 .mu.m for short-haul optical communications, and VCSEL technology at 0.8-1.00 .mu.m has matured to a point where it is possible to build VCSEL-based WDM optical data links for short-haul optical interconnects.
In recent years, micromechanically tunable VCSELs [13] and monolithically integrated multiple-wavelength VCSEL arrays [14]-[17] have been reported by various groups for free-space or fiber communications. Some of these can be incorporated into the present invention. In particular, the anodic oxidation scheme [18] or postgrowth tuning scheme [17] can easily provide the required device density and arbitrary wavelength variation within a small region. These also allow one to perform very flexible and accurate cavity-mode adjustment. The following discussion focuses on the utilization of anodic oxidation in the production of multiple-wavelength VCSEL PIE arrays.
To illustrate the wavelength adjustment of VCSELs using the anodic oxidation approach, the well-established 0.98 .mu.m VCSEL design with pseudomorphic InGaAs/GaAs quantum wells (QWs) and AlGaAs/GaAs DBRs will now be discussed.
FIG. 1 provides an example of the aluminum and indium composition profiles for a multiple-wavelength bottom-emitting VCSEL [1]-[5]. The laser structure consists of a 32-period top p-mirror and a 19.5-period output n-mirror. The compositions for the AlGaAs quarter-wave layers are pure AlAs for the first 18 periods of the n-DBR and Al.sub.0.9 Ga.sub.0.1 As for the rest of the structure, except for the oxide-aperturing layer which is located in the first AlGaAs quarter-wave layer above the active region and has a 400 .ANG. Al.sub.0.98 Ga.sub.0.02 As layer in the middle for selective lateral oxidation. The active region consists of three 80 .ANG. InGaAs QWs with 120 .ANG. GaAs barriers in between. To achieve multi-wavelength VCSEL PIE arrays by anodic oxidation requires two epitaxial growths. The first growth stops at the GaAs phase-tuning layer and then cavity modes for the individual channels are adjusted by performing anodic oxidation on this phase-tuning layer prior to the growth of the rest of the top DBR mirror. In FIG. 1, the phase-tuning layer is the fourth GaAs layer above the 1-.lambda. cavity, where .lambda. is the Bragg wavelength of the DBRs. The effects of choosing different tuning layer locations are covered in the following discussion.
FIG. 2A shows the corresponding calculated lasing wavelength, while FIG. 2B shows the corresponding threshold gain per quantum well as a function of the location and thickness of the GaAs phase-tuning layer. The number, m, shown beside each curve denotes that the tuning layer is the m-th GaAs layer above the 1-.lambda. cavity. For the special case: m=0, the tuning layer is the 1-.lambda. cavity itself. When m&gt;0and if the GaAs tuning layer is 0.75-.lambda. (point A in the figure), it behaves just like a normal quarter-wave layer and the device will lase at the Bragg wavelength .lambda.. However, when the GaAs tuning layer is 0.5-.lambda. or 1.0-.lambda. (points B and C in the figure), this layer itself becomes a second cavity in the whole structure with the same cavity mode as the original 1-.lambda. cavity. As a result, there will be a splitting in energy levels .lambda. points B and C) due to the mode-coupling effect. The amount of splitting decreases as the cavity separation increases but the wavelength-tuning curve becomes more and more nonlinear. Theoretically, the tuning curve will gradually change from a straight line of slope 2.lambda./n.sub.S.sup.2 to a straight line of zero slope when m increases from 0 to .lambda.. Here n.sub.S is the refractive index of the cavity. In order to achieve equidistant wavelength separation, a small m value, such as m=0 or at most m=1, should be used. Under this circumstance, the freespectral range is wider so that the available wavelength span is also larger than the designs with a larger m. However, the overall variation in device threshold currents to achieve this maximum wavelength span will be much greater, as illustrated in FIG. 2B. Moreover, the tolerance in process control of the anodic oxidation also becomes tighter because of the steeper wavelength tuning rates. For current short-distance coarse WDM applications, it is possible to use the m=3 or even m=4 design because the channel spacing is still much wider than 4 nm, which is mainly limited by the receiver bandwidth.
In both FIG. 2A and FIG. 2B, the threshold characteristics of the adjacent higher-order and lower-order modes for the m=4 case (solid lines) are shown to illustrate the mode-coupling effect at the two ends of the free spectral range. This coupling effect caused not only nonlinearity but also an abrupt switch in device threshold and lasing wavelength. The observed systematic variation in threshold characteristics comes from the extending of optical field intensity from the central 1-.lambda. gain cavity into the GaAs phase-tuning layer when the cavity mode is detuned away from the Bragg wavelength of the DBRs. Another factor that also causes reduction in optical confinement factor is the shift of optical standing-wave peak away from the InGaAs QWs. Consequently, threshold current will have its lowest value at the Bragg wavelength and goes up in either direction of wavelength. In particular, when the tuning layer thickness is outside the (0.5.lambda., 1.0.lambda.) range, the adjacent modes have lower threshold gains than the central mode and thus will dominate the lasing behavior.
The mode splitting at the 1.0-.lambda. tuning layer thickness determines the maximum available wavelength span. FIG. 3 shows the maximum lasing wavelength span and the required maximum threshold gain per well as a function of m. The solid curves are for the design shown in FIG. 1; while the dotted curves and the experimental data (0) are for the special case when we increase the first GaAs layer below the 1-.lambda. cavity to 1.25-.lambda. to avoid exposing AlGaAs layers on the etched surface. In reality, however, devices will not lase if the required threshold gain is higher than what the active material can provide, especially when the tuning layer thickness is close to 0.5.lambda. or 1.0.lambda.. If the maximum optical gain is experimentally limited to only 2400 cm.sup.-1 from each In.sub.0.2 Ga.sub.0.8 As quantum well, the solid curve of the maximum wavelength span has to be reduced to the dash-dotted curve by excluding those wavelength ranges where the required threshold gain is beyond 2400 cm.sup.-1 per quantum well. The curves suggest that m=2 would be the optimal design for providing the widest wavelength-tuning span whose wavelength-tuning characteristics are not overly steep. The latter is an important consideration for the processing control of anodic oxidation.
The above analysis and designs can be applied to other material systems at various wavelength regimes as well. The principle issue is the capability of performing successful epitaxial regrowth over the phase-tuning layer which can be GaAs, AlGaAs, InAlGaAs, InGaAsP, or others. For example, low-Al-content AlGaAs is very likely to be the tuning layer for 0.85-.mu.m WDM VCSEL PIE arrays so that high-quality AlGaAsAlGaAs over-growth interface has to be achieved for the above mentioned regrowth method. However, we can also use dielectric DBR mirrors, transverse wet oxidation [17], or other techniques to vary the resonant wavelengths if the regrowth technology is not available.
In addition, modal noise is a well-known phenomenon for multimode fiber systems when a coherent light source is used. For a multimode WDM link, if single-mode VCSELs are used at the transmitter part, output intensity profile for each VCSEL channel at the other end of the fiber will be very likely centered in a few spots which are distributed over the 62.5 .mu.m-diameter core in an unpredictable manner. Moreover, these spots will move around the fiber core when the fiber is bent or vibrated somewhere. Therefore, broad bandwidth multimode VCSELs are recommended for multimode WDM systems [19] to produce a uniform intensity profile at the fiber output. In our designs, an oxide aperture extending only 1.0 .mu.m from the edges of VCSELs is created to reduce optical losses for the high-order modes to facilitate multiple lateral-mode operation.
(c) RCPDs in Optical Data Links
Narrow-band monolithically-integrated RCPD arrays are needed for real-time spectroscopic analysis or parallel demultiplexing of wavelength-encoded channels [9]-[12]. The key issue in design is to achieve high quantum-efficiency and narrow-band square-like photoresponse. Traditionally, there are two major approaches: one by RCPDs and the other by multiple-cavity dielectric filters combined with discrete photodetectors. Because of the large numerical aperture associated with multimode fibers, the RCPD approach is expected to be much better than the other to generate narrow-band photoresponse [1].
In view of the foregoing, a need exists for a direct coupled-multimode WDM data link with monolithically-integrated multiple-channel VCSEL and photodiodes arrays. The present invention satisfies that need, as well as others, and overcomes deficiencies in prior approaches.